1. Field of the Invention
The present invention relates to portable data terminal equipment having a communication function, and more particularly to a data interface circuit capable of connecting portable data terminal equipment to a public switched telephone network (PSTN) and an analog cellular network, thereby enabling data transmission and reception.
2. Description of the Prior Art
Modems are digital communication equipment (DCE) for transmitting and receiving data between data terminal equipment by use of channels in the audio band. Such data terminal equipment is usually a personal computer.
FIGS. 1A and 1B are views for explaining the conventional concept of data communications in a data terminal equipment wherein data transmission and reception are achieved using PSTN's. Data terminal equipment such as personal or portable computers carry out data transmission and reception with other personal or portable computers via a modem serving as DCE and PSTN. Used as an interface technique between the data terminal equipment and the DCE is V.24, which is a standard scheme of Comite Consoltatif International pour Telegraphie et Telephonie (CCITT) typically called "RS232C". A data access arrangement (DAA) is used as an interface between the DCE and the PSTN, as in general telephones. As shown in FIG. 1B, the modem, which is the DCE, includes a modem control circuit 1 and an analog interface circuit 2. The DAA includes a hybrid circuit 3 and a line transformer 4 and serves to protect terminal equipment connected to the PSTN from a surge voltage. In this case, the hybrid circuit 3 is an impedance matching circuit for separating a transmitting signal Tx and a receiving signal Rx on a transmission line from each other. The line transformer 4 serves to optimize a signal TIP transmitted from the data terminal equipment to the PSTN and a signal RING received from the PSTN to the data terminal equipment for an impedance matching.
Recently, utilization of data transmission and reception using radio networks, such as cellular networks, has been abruptly increasing. In this case, transmitting-side and receiving-side cellular phones carry out data transmission and reception with each other by a trunking operation of a base station, as shown in FIG. 2A. Since radio channels are used in this case, as different from the PSTN shown in FIG. 1, modems, which are DCE's, should be constructed to transmit and receive radio frequency signals. Each modem should also be interfaced with the associated cellular phone executing a cellular protocol. Generally, the interface between the DCE and the cellular phone includes a hybrid circuit 3 and a line transformer 4. Additionally, this interface also includes an Rx/Tx separating circuit 5. This Rx/Tx separating circuit 5 serves to separate the transmitting signal Tx and the receiving signal Rx on the transmission line from each other, similar to the hybrid circuit 3 used in the DAA. It is advantageous that the Rx/Tx separating circuit 5 is provided with an operational amplifier because input and output signals of cellular phones are generally single-ended signals. As shown in FIG. 2B, signals on nodes A are used in this case, taking into consideration an impedance matching of the hybrid circuit 3.
In the above-mentioned conventional interface circuit, however, a signal attenuation occurs because the additional Rx/Tx separating circuit should be used to connect the modem and the cellular phone.. As a result, there is a limitation on data transfer rate. It is also difficult to provide various services because the interface circuit does not take into consideration the establishment of audio application path at all.